Insulin-dependent diabetes is a major health problem in the U.S. as well as in many other countries. The disease is characterized by a specific disappearance of the pancreatic B cells leading to a life-long dependence on daily insulin injections. The etiology and pathogenesis are unclear but both viruses or other environmental factors may be able to induce the disease against a background of the major histocompatibility antigens HLA-DR 3 and/or 4. It is of considerable interest therefore that newly diagnosed diabetic patients have autoantibodies reacting with pancreatic B cells. The antibodies, which may be present before the clinical onset of the disease, can be detected in a variety of islet cell preparations. However, current assay systems are cumbersome and qualitative. Furthermore, the antigens detected in these assays are not known nor is it known how different assay systems relate to each other or their predictive value for diabetes. Using radioactively labelled proteins from human or rat pancreatic islet cells it was demonstrated that sera from human diabetic patients or spontaneously diabetic BB rats detect a major antigen being a Mr 64000 protein. This protein will be isolated and characterized to determine its structure and reactivity with autoantibodies in diabetic sera. Techniques of gelelectrophoresis, electroelution of protein components and analysis of amino acid sequences will be used to determine the primary structure. Long term objectives include determination of the complete structure by molecular cloning and use of the isolated protein or synthetic peptides thereof to develop quantitative and reproducible assay for autoantibodies associated with onset of insulin-dependent diabetes. Knowledge of an islet cell antigen may prove useful to explain the mechanisms by which the immune response in certain individuals is directed against self-pancreatic B cell antigens.